V. C. Illing—The Search for Oil. 297 



If it is expected that the British Carboniferous strata will contain 

 underground oil-pools, it will be legitimate to assert that the same 

 general laws which are found essential for the occurrence of oil-pools 

 in the American Palaeozoic fields, ought to be applicable with the 

 same force to the Carboniferous strata in Britain. It will then be 

 advisable to examine the general tectonics of the American Palaeozoic 

 fields in order to discover the structural laws governing the occurrence 

 of the oil, and afterwards to apply these laws to our own Upper 

 Palaeozoics in Britain. 



The Pennsylvanian oil and gas fields will be taken as a typical 

 area which has been thoroughly tested with the drill, and which, 

 moreover, can be readily studied by means of the excellent maps and 

 reports published by the United States Geological Survey. The 

 structure of these fields is in the main a broad flat geosyncline of 

 Devonian and Carboniferous rocks bordering the western edge of the 

 Pennsylvanian Mountains. The beds are entirely devoid of true 

 folding, but are warped into an alternating series of structural 

 terraces or curved into a succession of extremely gentle undulations. 

 These sinuous undulations are termed anticlines and synclines, but 

 it must be remembered that their dips are usually only from 20 to 

 50 feet in the mile and seldom approach 2°. 



The structures throughout the oil and gas field belt are all of this 

 gently undulating, almost horizontal type. It is significant that 

 where the productive zone approaches the real zone of folding on the 

 east, where the sinuous broad undulations are replaced by definite 

 anticlines and synclines with dips of 5° and upwards, the oil and gas 

 suddenly disappear, though the same strata still persist in this outer 

 belt of folded rocks. Thus in S.W. Pennsylvania the gas anticline of 

 Belle Vernon is succeeded on the east by the Brownsville anticline 

 with a small oil and gas field at its southern extremity. Both these 

 structures are merely gentle undulations with dips below 1°. The 

 succeeding anticline on the east, the Favette anticline, is a definite 

 fold structure with dips of 3° to 4°. Here the prolific oil and gas 

 zone has almost completely disappeared, and only a few gas wells of 

 small yield lie on this otherwise completely barren anticline. Still 

 further east no oil or gas has been discovered. 



This sudden impoverishment of the strata takes place within 

 a distance of 5 to 10 miles, and it can be traced longitudinally along 

 the border of the folded zone for over 100 miles. The position which 

 marks the eastern limit of the main oil and gas fields is found in 

 every case to coincide with the first anticline where the dips approach 

 or exceed 3° to 4°. 



It may be objected that these statements are a direct negation of 

 the well-established theory of the anticlinal occurrence of oil, 

 a theory which has proved its value all the world over. In spite 

 of the recent tendency in oil technology to overestimate the 

 importance of this idea, there is no doubt that it constitutes one 

 of the fundamental principles of oil concentration. It is not 

 suggested by the writer that the oil in Tertiary fields cannot and 

 does not occur in strongly developed anticlinal folds, but it is 

 asserted that the productive fold structures of the American 



